Homogeneous nickel-base superalloy castings

ABSTRACT

D R A W I N G HOMOGENEOUS CASTING ARE ACHIEVED IN THE NICKEL-BASE SUPERALLOY SYSTEMS BY THE SUPPRESSION OF DENDRITIC SEGREGATION IN CONTROLLED UNIDIRECTIONAL SOLIDIFICATION, THE CASTING BEING CHARACTERIZED BY CELLULAR AND PLANE FRONT MICROSTRUCTURES.

July 18, 1972 J, K. TIEN ETAL 3,671,835

HOMOGENEOUS NICKEL-BASESUPERALLOY CASTINGS Filed Oct. 16, 1970 2 Sheets-Sheet 1 W6. Z ZV O-Ooooda g l 0 au 5am/099V o /00 20a @'00 42630 5&0

ST/Q/r/ff ffm/R554 /f/fa/VJ July 18, 1972 J. K. TIEN ETAL HOMOGENEOUS NICKEL-BASE SUPERALLOY CASTINGS 2 Sheets-Sheet 2 Filed Oct. 16, 1970 z Q l.. llll I l l l l l ll W l\l\\l\\l\l\. KN l UI a @l UKQQ. l n MAYYMQ n ww wwmklw //Y 332729Z@ @7.5% ZZ@ United States Patent O 3,677,835 HOMOGENEOUS NICKEL-BASE SUPERALLOY CASTINGS John K. Tien, Rocky Hill, and Robert P. Gamble, Killing- Worth, Conn., assignors to United Aircraft Corporation, East Hartford, Conn.

Filed Oct. 16, 1970, Ser. No. 81,229 Int. Cl. C22c 19/00 U.S. Cl. 14S-32.5 4 Claims ABSTRACT OF THE DISCLOSURE Homogeneous castings are achieved in the nickel-base superalloy systems by the suppression of dendritic segregation. in controlled unidirectional solidication, the castings being characterized by cellular and plane front microstructures.

BACKGROUND OF THE INVENTION The present invention relates in general to the nickelbase superalloy iield and, more particularly, to such alloys as processed to provide castings characterized by a cellular or plane front cast microstructure and the substantial absence of dendritic segregation.

The nickel-base superalloys are, of course, recognized as those alloys particularly adapted to high temperature operation in demanding environments such as those associated with the operation of gas turbine engines. Typically, they consist of a nickel chromium solid solution matrix ('y phase) strengthened by aluminum and titanium in the form of a precipitate usually represented by the formula Ni3(Al,Ti) ('y' phase). In addition they also normally include cobalt and the refractory metals for solution strengthening and often carbon, boron and zirconium.

Representative of such alloys are the following:

Description: Composition (by weight) Udimet 700 15% Cr, 18.5% Co, 3.25% Ti, 4.25% Al, 5%- Mo, .1% C,

.03% B, balance Ni. B-l900 3% Cr, 10% Co, 1% Ti, 6% Al, 6% Mo, .11% C, 4.3% Ta, .07% Zr, .15% B, balance Ni. 9% Cr, 10% Co, 2% Ti, 5% Al, 12.5% W, .15% C, 1% Cb, 0.15% B, .05% Zr, balance Ni.

A common feature of all current nickel-base superalloy cast structures is the presence of dendritic segregation. Recently, through controlled unidirectional solidiiication techniques, the extent of constitutional inhomogeniety and other deleterious structural manifestations of dendritic segregation has been significantly reduced, or controlled, resulting in more homogeneous cast structures. Thus, the ancient art of structural manipulation of metals has received new impetus through these recent advances in certain casting processes. One of these recent advances in directional solidication is disclosed by Ver Snyder in U.S. Pat. No. 3,260,505 providing aligned grain boundaries in superalloy castings, and by Piearcey in U.S. Pat. No. 3,494,709 -wherein grain boundaries are eliminated.

While the ensuing columnar grained and monocrystalline castings possess near optimum grain morphology and superior overall properties for elevated temperature applications, complete elimination of dendritic segregation would be additionally advantageous. Depending on the particular superalloy chemistry such segregation can result in the formation of low melting or brittle phases, nonuniform distribution of the strengthening precipitates, interdendritic porosity and, surface freckles. In a specic application, one or more of these structural manifestations of dendritic segregation can be undesirable.

MAR-M200 Cil 3,677,835 Patented July 18, 1972 ICC In a number of alloy systems the means for eliminating or minimizing dendritic segregation are traditionally dependent upon either a solid state diffusion or mechanical working of the casting. However, the nickel-base superalloys, and other constitutionally complex alloys, strongly resist solid state homogenization, and under no circumstances is porosity eliminated by heat treatment. Furthermore, mechanical working negates the advantages of shape casting and oriented grain structures, either columnar or single crystal.

SUMMARY OF THE INVENTION This invention contemplates the achievement of homogeniety of structure in the constitutionally complex alloys, such as the nickel-base superalloys, by the suppression of dendritic growth, and manifested in the achievement o cellular and plane front cast microstructures.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a graphic representation of the degree of tungsten segregation normal to the growth direction in dendrictic crystals of MAR-M200.

FIG. 2 is a similar graphic representation for cellular/ plane front crystals in the same alloy system.

FIG. 3 depicts the high cycle fatigue limit of dendritic material with and without carbides as contrasted with plane front material without carbides.

FIG. 4 is a graph comparing the tensile yield strength and ductility of MAR-M200 single crystals, contrasting the properties of dendritic and cellular/ plane front structures.

DESCRIPTION O-F THE PREFERREDy EMBODIMENTS The approach utilized herein contemplates the suppression of dendritic :growth during solidiiication to provide unique cast nickel-base superalloy structures displaying significant improvements in physical properties. The resulting cellular and plane front cast structures have never before been achieved in these alloys to the best of the inventors knowledge.

The MAR-M200 alloy was selected as representative of one of the more constitutionally complex nickel-base superalloys and, inasmuch as it has been extensively investigated in columnar grained and monocrystal form, properties are readily available for the purposes of comparison. Accordingly, castings to the MAR-M200 chemistry were unidirectionally solidified according to the present invention in a vertical crystal grower of the type described in the aforementioned Ver Snyder patent, modified to provide positively controlled motion of the soliditication front. This was accomplished through the programmed movement of a single turn induction coil upward of the casting mold. As a matter of practice the coil was utilized to provide localized heating with portions of the ingot above and below the localized heating zone in a solidified state. With effective zone melting of this nature better casting constitutional homogeniety was felt possible. However, solidication upward from the melt is also perfectly feasible.

In the Ver Snyder process, as disclosed in the patent, columnar grain growth is achieved by removing heat from the melt through a water-cooled chill plate at the bottom of the mold while the power to the heating coils is o. This is, of course, unidirectional solidilication. In the Ver Snyder process dendrites are formed by upward growth in a so-called mushy zone which precedes the advancing solid/melt interface and is perhaps 3-9 inches in depth, depending upon the system parameters.

The homogeneous castings of the present invention have been provided by severely curtailing the depth of the mushy zone to inhibit the dendritic growth occurring therein. For this purpose, the single turn induction coil is utilized to establish a high thermal gradient, typically 300-900 F. at the solid/melt interface, limiting the mushy zone to a maximum of about 1/2 inch in depth in this apparatus. A programmed upward movement of the coil at a rate of about 0.2 inch/hour, with a corresponding upward velocity of the interface, has been formed to provide the cellular and plane front structures sought.

In general, in the solidcation process categorized by slow soliditication rates, the higher the thermal gradient the higher the soliditication rate achievable, particularly in the apparatus described. This is true since the high gradient, of course, limits the extent of dendritic growth, if any, by providing a temperature in excess of the liquidus temperature of the alloy at or very close to the interface. It will also be recognized that the dendritic segregation problem may also be regulated by limiting the time available for dendritic growth.

In a slow solidication rate process, it has been established, as previously mentioned, that the mushy zone must not exceed about 1A. inch in depth. Typically, the aim depth will be 1 centimeter or less and it has been conclusively established that at a Adepth of l centimeter a cellular structure is provided and at a mushy zone depth of 1 millimeter a plane front structure is achieved.

There appears to be no size limitations on the castings attainable and columnar grain and single cnystal articles up to 1 foot in length have been fabricated, normally for convenience in molds 1/2-1 inch in diameter.

The unique cast structures Idescribed exhibit significantly improved physical properties even when compared to the advanced columnar-grained and monocrystalline articles obtained as taught by Ver Snyder and Piearcey. In the MAR-M200 system, these improvements include a substantial enhancement of ductility, significant improvement in fatigue properties, higher incipient melting temperature and elimination of surface defects (freckles) from single crystals.

The marked difference between the normal dendritic cast structure and the more homogeneous cellular and plane front cast structures is readily ascertained by differential etching and this technique has been utilized in the examination of transverse sections of MAR-M200 single crystals. In the ydendritic cast structure, pronounced segregation is revealed as an array of light colored dendrite crosses surrounded by a large amount of darker interdendritic material. The cellular structure is essentially homogeneous except for narrow regions at the intercellular boundaries which act as traps for dispersed phases which form in the melt during solidtication, in this case approximately 1 volume percent MC-type carbides. In contrast the plane front structure achieved with MAR- M200 with very low carbon concentration is entirely homogeneous. Thus, it will be recognized that impurities as well as certain constituents such as carbon are important in determining whether a given homogeneous cast structure will be cellular or plane front.

FIGS. 1 and 2 show the degree of tungsten segregation normal to the growth direction in dendritic and cellular/ plane front crystals of MAR-M200 as ascertained by a microprobe scan. The scan data are normalized relative to the tungsten concentration in the center of the dendrites or cells, respectively. In the dendritic structure, there is a 50 percent difference in tungsten content between dendritic and interdendritic regions, reilecting dendritic segregation. On the other hand, in the cellular/plane front structures, the tungsten distribution is nearly homogeneous throughout the section. Obviously, all other alloy additions must also be hornogeneously distributed since tungsten, with its low diiusivity, is the most ditlicult element to homogenize.

There is also an ascertainable difference in the fy precipitate morphology in the dendritic and cellular/plane front single crystals. The dendritic material has a nonuni- 4 form 'y' size and distribution with finer 'y' precipitates occurring in the dendrites (high tungsten, low aluminum and titanium) and large 'y' precipitates in the interdendritic regions (low tungsten, high aluminum and titanium). The cellular/ plane front structures, however, exhibit a uniform fy' size and distribution.

One benefit of a uniform 'y' size is an expected improved creep resistance resulting from the increased phase stability associated with precipitates of uniform spatial morphology. This is of extreme importance in high volume fraction 'y' alloys such as NX-188 (8 Al, 18 Mo, balance Ni) wherein the 'y' morphology is typically extremely heterogeneous because of dendritic segregation.

Still another :advantage of the homogeneous cast structures provided hereby is that the low melting Fy' phase (also often referred to as eutectic or massive 7') normally associated with the interdendritic regions is not present in the cellular/plane front castings. The elimination of the low melting 7', a nonequilibrium phase, is the inherent result of the more uniform distribution of the 'y' forming elements in the cellular/plane front cast structures. In MAR-M200 this elimination of the low density melting phases raises the incipient melting point of the alloy at least 75 F., from 2265 iF. to 23400 F.

Carbides forming during casting are not nonequilibrium phases and are present even in homogeneous cast structures. However, the deleterious eli'ects thereof, when present, are greatly reduced in the cellular/plane front crystals, the `MC carbides exhibiting precracldng in the dendritic structure but not in the castings of the present invention.

Normally associated with dendritic solidiiication is the entrapment of evolved gases at the solid-liquid interface. Since the homogeneous castings are evolved with an essentially unperturbed solid-liquid interface during growth there are no dendrites and therefore, no resulting gaseous porosity. And the suppression of gaseous porosity in homogeneous cast structures is proving as significant in contributing to fatigue resistance as the elimination of precracked carbides.

The tensile strength, ductility and fatigue data presented in FIGS. 3 and 4 compare the properties of the old and new structures. There is a four-fold improvement in the ductility of cellular MAR-M200 single crystals over that of dendritic crystals between room temperature and 1400 F.

In view of the foregoing it is believed that the foregoing clearly establishes the significant advance in the superalloy art provided by the unique materials and techniques herein described. While it has been convenient to describe the invention in detail in connection with certain preferred embodiments and examples, these will be runderstood to be illustrative only and no limitation is intended thereby. Numerous modifications and improvements will be evident to those skilled in the art from the teachings herein and the invention in its true spirit and scope will be measured in accordance with the claims hereinafter set forth.

What is claimed is:

1. Unidirectionally solidified castings composed of an alloy consisting essentially of, by weight, up to about 25 percent chromium, 4-10 percent selected from the group consisting of aluminum, titanium and columbium, up to A3-0 percent cobalt, 3-20 percent selected from the group consisting of molybdenum, tantalum and tungsten, up to about 5 percent selected from the group consisting of boron, zirconium, hafnium and carbon, and at least 45 percent nickel, exhibiting a cellular/plane front microstructure and characterized by the substantial absence of dendrites.

2. `Castings according to claim 1 further characterized by a columnar grained microstructure.

3. Castings according to claim 1 further characterized by a monocrystalline microstructure.

4. In the production of columnar grained and monocrystalline castings from the nickel-base superalloys consisting essentially of, by weight, -up to about 25 percent chromium, 4-10 percent selected from the group consisting of aluminum, titanium and columbium, up to about 30 percent cobalt, 3-20 percent selected from the group consisting of molybdenum, tantalum and tungsten, up to vabout 5 percent selected from the group consisting of boron, zirconium, hafnium and carbon, and at least 45 percent nickel, the improvement which comprises unidirectionally solidfying the superalloys from the melt in a cellular/plane front solidication mechanism limiting the depth of the mushy zone preceding the advance of the 7/1966 Ver Snyder 75-171 -2/ 1970 Piearcey 75--171 RICHARD O. DEAN, Primary Examiner U.S. Cl. X.R. 

